AVS 57th International Symposium & Exhibition
    Thin Film Wednesday Sessions
       Session TF-WeA

Paper TF-WeA12
Influence of Growth Rate on the Epitaxial Orientation and Crystalline Quality of CeO2 Thin Films Grown on Al2O3(0001) by Oxygen Plasma-Assisted Molecular Beam Epitaxy

Wednesday, October 20, 2010, 5:40 pm, Room Pecos

Session: Thin Films: Growth and Characterization
Presenter: M. Nandasiri, Pacific Northwest National Laboratory
Authors: M. Nandasiri, Pacific Northwest National Laboratory
S. Kuchihbatla, Pacific Northwest National Laboratory
P. Nachimuthu, Pacific Northwest National Laboratory
T. Varga, Pacific Northwest National Laboratory
V. Shutthanandan, Pacific Northwest National Laboratory
W. Jiang, Pacific Northwest National Laboratory
S. Thevuthasan, Pacific Northwest National Laboratory
S. Seal, University of Central Florida
A. Kayani, Western Michigan University
Correspondent: Click to Email
Cerium oxide thin films were grown on Al2O3(0001) substrates with different growth rates (1-10 Å/min) by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The growth rate induced epitaxial orientations and crystalline quality of CeO2 thin films were studied by in-situ reflection high-energy electron diffraction (RHEED), ex-situ atomic force microscopy (AFM), and high-resolution x-ray diffraction (HRXRD) techniques. CeO2 grows as three-dimensional (3-D) islands and two-dimensional (2-D) layers at growth-rates of 1-7 Å/min and ≥9 Å/min, respectively. The average surface roughness of 5-10 Å shows high-quality surfaces of CeO2 thin films. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 Å/min and ≥ 9 Å/min, respectively. Glancing incidence XRD measurements have indicated that the films grown at intermediate growth rates (2-7 Å/min) consist of some polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 Å/min showed six in-plane domains, characteristic of well-aligned CeO2(100) crystallites. All six of the repeating rectangle units of O atoms from the oxygen sub-lattice in Al2O3(0001) that bind to Ce atoms are nonequivalent which produces six in-plane domains. This also minimizes the lattice mismatch between the thin film and the substrate leading to well-aligned CeO2(100) crystallites. When increasing the growth rate from 1 Å/min to 2-7 Å/min, the lack of sufficient time to stabilize the Ce atoms on all the rectangle units of O atoms from oxygen sub-lattice in Al2O3(0001) results in poorly-aligned CeO2(100) crystallites that start to coexist along with well-aligned crystallites. Furthermore, the content of the poorly-aligned CeO2(100) crystallites increases with increasing growth rate up to 7 Å/min, and three out of six in-plane domains gradually decrease and eventually disappear. At growth rates ≥9 Å/min, CeO2(111) film with single in-plane domain was identified. In order to accommodate the CeO2(111) unit on top of the Al2O3(0001), the cerium sub-lattice undergoes compression in all three axes by ~24% resulting in poorly-aligned CeO2(111) crystallites. The formation of CeO2(100) 3D-islands at growth rates of 1-7 Å/min is a kinetically-driven process unlike at growth rates ≥9 Å/min which result in an energetically and thermodynamically more stable CeO2(111) surface.